Analytical Corrections for Acoustic Transmission in Kevlar-Walled Wind Tunnels

Kevlar-walled aeroacoustic wind tunnels have become important research facilities to evaluate low-noise wind turbine designs. In this paper a semi-analytical Green's function for the transmission of sound through Kevlar walls and shear layers was developed in order to provide accurate acoustic measurements at high flow speeds. The semi-analytical Green's function was evaluated experimentally with a monopole sound source. It was possible to find the correct source location when the Green's function was used to formulate the steering vectors for the beamforming technique. The corrected steering vectors provided a much better source level estimation than if a free-field Green's function was used to formulated the steering vectors, but there was a systematic overcorrection at low frequencies and an undercorrection at high frequencies. The analytical transmission loss was very sensitive to the convection velocity of the vorticity shed at the membrane apertures, which in turn depended on the aerodynamic loading of the Kevlar walls. It is likely that further fine-tuning of the formula of the convection velocity will reduce the systematic error. The experimental setup to validate the Green's function could be improved if aerodynamic loading was applied to the walls.